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post #1 of 18 (permalink) Old 02-15-2015, 08:52 AM Thread Starter
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Default Confused about bushes

Hi,

I'm making a router table for my dewalt 625EK. Following Bill Hynton in principle.

I've made the mounting plate and the top and now need to make the cutout in the top for the mounting plate.

So I used a 10mm cutter and a 17mm guide bush. (I only have a 17mm)

Now reading and doing a few simple drawings if I make the internal template using the above the combination and then use the same combination for the cut out in the table top, the offsets etc should be the same. ie the inside template hole should be 13.5mm larger than the mounting plate and then the cutout in the top should be 13.5mm smaller,

I made the internal template but when I measure it up against the mounting plate I find it's 2mm out.

Am I making the wrong assumptions about the offsets or this a skill issue?

Many thanks

Andy
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post #2 of 18 (permalink) Old 02-15-2015, 11:49 AM
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I get confused on this sometimes, and find that it's easier to make a quick sketch. Assuming that both templates are female, I think that the sizes would be per the attached sketch. What gets confusing is that you're cutting on opposite sides of the cutter, depending on which part you're making - hopefully I've got the math correct (on the second attempt).
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File Type: pdf Template offsets.pdf (91.4 KB, 140 views)
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post #3 of 18 (permalink) Old 02-15-2015, 12:26 PM
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Hi Andy

In metric you don't get the nice relationship ratios between measurements that you do in Imperial (one of the very few advantages of Imperial TBH). I've just revistied this - to do this in metric with a 10mm cutter and standard (off the shelf) guide bushes you'd need to make the first cut using a 20mm guide bush on the inside of the template, then swap to a 40mm guide bush with the 10mm cutter for the outside cut. Which will work

With Imperial your first cut needs to be something like 1/4 in cutter with 3/8in guide bush followed by 3/8in cutter with 1in guide bush.

Regards

Phil

"Unfortunately there is lots of bad information online; some of it is really scary. It's probably not intentional, but I've seen some content that sets up the illusion that you can do whatever you want and get away with it" - Norm Abram in an interview with Jefferson Kolle

Last edited by Phil P; 02-15-2015 at 01:09 PM.
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post #4 of 18 (permalink) Old 02-15-2015, 12:38 PM Thread Starter
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Hi, tomp913

I've looked at your sketch and I realised the error of my way. I've been thinking about all wrong. I haven't checked your maths but now you given me a good pointer I'll re-try my own calculations.

Also since my OP I've noticed that I was using a 12.7mm cutter so nothing would have added up. Luckily I did trial cuts.

So a new sketch and new calculations after tea.

Many thanks

Andy
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post #5 of 18 (permalink) Old 02-15-2015, 01:38 PM
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Andy this drove me nuts at first ,as I forgot to take into consideration the bit width

I did a little video to help me understand the error of my ways

http://youtu.be/bBTrI15ad04

I donít always insulate , but when I do .
Ok ,I never insulate

Last edited by RainMan 2.0; 02-15-2015 at 01:42 PM.
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post #6 of 18 (permalink) Old 02-15-2015, 01:59 PM
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Default Easy Jig to cut Top

Andy,

If you have already made the mounting plate, and have a pattern bit with the top bearing, here's an easy way to make an exact size cutout in the table top - fix the mounting plate in place, snug the strips around it as shown and cut around the inside of the strips for a (hopefully) perfect fit for the mounting plate. This is what's shown/recommended for those who use a "store bought" aluminum mounting plate. The plus is that no specific template is needed for the table cutout.

Tom
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File Type: pdf easy jig to cut top.pdf (168.4 KB, 87 views)
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post #7 of 18 (permalink) Old 02-16-2015, 06:49 AM Thread Starter
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Hi All,

with out replying to specific posts:
I thank you for your assistance.
I can't get a 40mm bush readily (£8.50) but 32mm (£2.50) is readily available and I need to check If I have a 10mm bit. ( I though that was what I was using but not as it turns out).

Having read your info I think I can produce a spread sheet for my calculations and pick a bush and tool. I need to buy some more because most of the bits I have are old, worn and HSS and I only have the one 17mm bush supplied with my MOF96.

I like the idea of using strip tacked round the mounting plate. The only fly in the ointment is that I radiused the corners of the mounting plate using a coin so I may have troubles get a tool of the correct size so I don't get them filled with dust and chippings.


I'll let you know how I get on. It may not be for a week or so because I have to fit a cooker hood and a bathroom extractor next week end.


Cheers
Andy

I've just done a spread sheet and the difference between the bushes doing an internal and external cut using the same tool is twice the tool diameter. I need to make the spread sheet a little more explanatory before publishing.

Last edited by AndyNC; 02-16-2015 at 07:04 AM. Reason: More understanding
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post #8 of 18 (permalink) Old 02-16-2015, 09:42 AM
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This might help - you'll need to use a conversion chart (which is easy to find on the web) to solve your problem:

The most important thing to learn about using bushings is the offset—the distance between the cutting edge of the router bit and the outside edge of the bushing.
The offset is the key to creating jigs and templates to cut the sizes and shapes you want. To determine the offset, subtract the bit diameter from the bushing diameter and divide by two. For example, with a 3⁄4-in.-dia. bushing and a 1⁄2-in.-dia. bit, the difference is 1⁄4 in. Half that, or 1⁄8 in., is the offset, so the edge of the template must be 1⁄8 in. from the edge of the cut.

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post #9 of 18 (permalink) Old 02-16-2015, 01:09 PM
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Hi can anybody tell me what is the most used makita 3612 s guide bush . I would like to thank everybody for all help I recived. Jim
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post #10 of 18 (permalink) Old 02-16-2015, 03:34 PM
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Quote:
Originally Posted by Jimwarren View Post
Hi can anybody tell me what is the most used makita 3612 s guide bush . I would like to thank everybody for all help I recived. Jim
Probably 30mm, Jim, because that's the one you use with kitchen worktop jigs

Regards

Phil

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